The present invention is directed to an optical waveguide that provides both weak and strong photon confinement in a unitarily formed waveguide device.
Optical data transmission offers various advantages over other forms of data transmission, primarily with regard to bandwidth and size of the transmission medium (e.g., fiber-optic cables, waveguides, etc.). Additionally, recent developments have made more attractive the fabrication of integrated optical devices suitable for use in optical data transmission systems. Examples of such developments can be found in U.S. Pat. Nos. 5,790,583; 5,825,799; 5,878,070; 5,926,496; and 6,009,115, the contents of each of which are incorporated by reference herein. Those references describe various optical devices such as lasers, resonators and wayeguides, which are well-suited for use in construct ing data and telecommunication optical networks.
Heretofore optical networks have routed or otherwise controlled the transmission of light (i.e., of an optical signal) by converting the light signal into an electrical signal, manipulating the converted electrical signal using electronic components, and then converting the electrical signal back into a light signal. Such conversion-intensive signal processing is, however, undesirable because it slows and complicates data flow.
It is therefore desirable, whenever a light signal is to be manipulated, to avoid converting light signals to electrical signals. Rather, it is preferable to instead use optical devices to manipulate the light signal directly, and thereby simplify and speed operation of the optical network. Eliminating many of the electronic components from optical networks also facilitates the integration of very small (i.e., nanometer scale) optical components in the optical networks. In some cases such optical components may comprise a plurality of integrated devices formed on a single substrate much in the same manner as the integrated electrical semiconductor devices which are today in widespread use.
The waveguides currently used in optical networking may vary in their size and construction because different waveguide configurations are preferred for different uses. A new generation of optical waveguide devices now employed in optical data systems uses nanostructure (i.e., nanometer scale) deeply etched waveguides to control light pulses. Such nanostructure deeply etched waveguides strongly confine the light transmitted therein, and offer benefits such as reduced overall linear insertion losses, and maximized optical power coupling efficiency into the nanostructure waveguides. Other optical components may include waveguides which weakly confine the light transmitted therethrough, such as, for example, shallow etched waveguides. By way of example, conventional shallow etched waveguides transmit light efficiently and so are suited for use whenever light is to be sent a substantial distance.
For various reasons dictated by the laws of optics, it is eventually preferable to transmit an optical signal through weakly-confining, rather than strongly-confining waveguides. Such weakly-confining waveguides are known, and may be generally characterized as two-dimensional strip waveguides. Weakly-confining waveguides typically have a core width of at least 2 xcexcm. In contrast, strongly-confining waveguides may be deeply etched and have a width of not more than 1 xcexcm. The deeply-etched structure of such waveguides minimizes leakage of optical power carried by the tail of the guided mode into the substrate.
Although nanostructure optical devices employ nanostructure deeply etched waveguides, the light pulses eventually will, because of signal transmission issues, pass into weakly-confining conventional shallow etched waveguides, which have lower propagation losses than strongly-confining waveguides. Such weakly-confining waveguides may take the form of shallow etched waveguides and are preferable for transmission of light pulses because they are single mode, and because they are relatively easy to fabricate. Arranging for the efficient passage of light between the two types of waveguides is, however, difficult. For example, light transmitted between weakly-confining and strongly-confining waveguides will be subject to losses, such as reflection loss, which occurs when light propagates from one waveguide to another.
Although light can be transferred from a conventional weakly-confining strip waveguide to a nanostructure deeply etched waveguide at a butt joint, such a connection is undesirable because it is subject to losses. The small cross-section of the nanostructure deeply etched waveguide makes its coupling efficiency to the conventional weakly-confining strip waveguide poor. This occurs because the required deep etch of the nanostructure strongly-confining waveguide makes such a structure multi-mode, while the weakly-confining waveguide is single-mode. This means that a significant part of the coupled optical power transmitted into these sections will be carried by the higher order modes and will be radiated when it arrives at the devices which are served by the nanostructure deeply etched waveguide. This effect increases the linear insertion loss of such devices.
The term xe2x80x9cwaveguidexe2x80x9d will be understood by those skilled in the art to refer to optical components having a core of material surrounded by cladding, with both the core and cladding being transparent to light and having a respective index of refraction. The core may be a buried structure, in which case it is completely surrounded by cladding. Alternative, the core may be a ridge or strip structure, in which case it is partially surrounded by cladding, and partially surrounded by another medium such as, for example, air or a vacuum having respective index of refraction.
To xe2x80x9cstrongly-confinexe2x80x9d generally refers to a difference in refractive indices (xcex94n) between the waveguide core, cladding, and surrounding medium (if provided) of at least a particular amount. To xe2x80x9cweakly-confinexe2x80x9d refers to a waveguide in which the difference in refractive indices between the waveguide core, cladding, and surrounding medium (if provided) is less than that particular amount.
A waveguide may be a photonic-wire waveguide, which provides a waveguide core surrounded in all directions transverse to photon propagation direction, such as, for example, both in a width and thickness direction, by a relatively low refractive index (compared with the core) medium such as air, silica, or other relatively low refractive index material, to provided strong photon confinement in all directions perpendicular to their propagation direction in and through .the waveguide core. A waveguide may also be a photonic-well waveguide, which provides a waveguide core surrounded on opposite sides in a direction transverse to photon propagation direction, such as, for example, in a width direction, by a relatively low refractive index medium or material, to provide strong photon confinement in a direction perpendicular to their propagation direction in and through the waveguide core.
Thus, there exists a need in the art for an optical component that overcomes the above-described shortcomings of the prior art. In particular, there is a need for devices which increase the coupling efficiency between weakly-confining and strongly-confining waveguides, and which reduce insertion losses at such junctions by decreasing the scattering loss from the side walls of the input and output sections which is due to a shallow etch.
The present invention is directed to a novel waveguide structure that provides, in a unitarily formed waveguide, weak photon confinement and strong photon confinement along a propagation direction defined by a core through the waveguide.
In an embodiment of the present invention, an optical waveguide through which an optical signal may propagate in a propagation direction and along an optical path comprises a first waveguide section providing weak confinement of the optical signal in a direction generally transverse to the propagation direction and a second waveguide section providing strong confinement of the optical signal in all directions relative to the propagation direction. A tapered neck is provided between the first and said second waveguide sections and a core is defined through the first and second waveguide sections and the tapered neck, and through which the optical signal may propagate in the propagation direction.
The present invention is also directed to a method of fabricating a waveguide having a weakly-confining waveguide section and a strongly-confining waveguide section optically coupled by a neck that simultaneously tapers in two directions.
The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the disclosure herein. The scope of the invention will be indicated in the claims.